In an X-ray computer tomography (CT) scanner, an object is positioned between an X-ray generator and an X-ray detector. The X-ray generator and the X-ray detector are rotated around the object, and the object is exposed to an X-ray beam generated by the X-ray generator beam in many directions. The X-ray intensity distribution transmitting the object (or a projection of the object) is measured with the X-ray detector. Based on the X-ray projection data acquired in one rotation, a distribution of linear absorption coefficients (or an image) inside the object is reconstructed two-dimensionally to create a slice image of the object. The reconstruction calculation is performed on a plurality of planes perpendicular to the rotary axis, to create a three-dimensional image based on the slice images.
If a magnification of an image can be changed in a CT scan, a field of view (a size of a region of interest) can be changed, and a resolution of the image can be changed. In an X-ray CT scanner having an X-ray generator and an X-ray detector fixed to a rotary arm or a gantry, the X-ray generator and the X-ray detector are rotated around an object while keeping the distances among the X-ray generator, the object and the X-ray detector constant. The magnification of an image can be increased or decreased according to the relative distance between the X-ray generator or X-ray detector and the object.
As will be explained later, a distance between the X-ray generator and a rotation center for CT scan and/or a distance between the X-ray detector and the rotation center can be changed relatively in this invention. This is relevant to some prior art documents. In a CT scanner described in Japanese Patent laid open Publication 2004-329293, transmission still images (called as scout images) are taken in two positional relationships between the X-ray generator and the X-ray detector before starting a CT scan, in order to determine a three-dimensional position of a region to be scanned. When the size of a scout image is changed, the position of a chair is moved relative to the X-ray detector. However, though the size of a scout image can be changed, a structure for changing the magnification of an image is not described. In a CT scanner described in Japanese Patent laid open Publication H5-322802/1993 to be used for an object in industrial fields, the positions of the X-ray generator and the X-ray detector are fixed. On the other hand, an object is put on a table, and a projection image of the object is taken while the table is rotated. The magnification of the image is changed according to the positions of the X-ray generator and the X-ray detector relative to the rotating table. However, the rotation of the object is possible because the object is a matter. When the object is a person, problems such as artifacts due to his or her motions and dizziness caused by the rotation occur. Therefore, it cannot be adopted practically for CT scan of a person, especially for medical areas. Further, even when the object is a matter, the CT scanner cannot be used if the object has a structure so precise as not to be subjected to continuous rotation. In an X-ray CT scanner described in Japanese Patent laid open Publication 2001-37747, a rotary arm has an X-ray generator and an X-ray planar detector opposing to each other, and an object is arranged between them. The X-ray planar detector is mounted to a ring arm with an extensible arm, and the detector can be set nearer or farther from the object so that a magnification of an image can be changed. However, a mechanism for moving the X-ray detector relative to the rotation device or for moving the rotation device relative to the X-ray device is necessary to change the magnification. Medical devices are used widely for panorama X-ray imaging or for X-ray CT scan wherein an X-ray generator and an X-ray detector are provided at two ends of a rotary arm, and it is desirable to modify them.